1. Field of the Invention
The present invention relates to speakers for use in an acoustic system, and in particular relates to a speaker having two voice coils arranged coaxially with diameters different from each other, and a manufacturing method therefor.
2. Description of the Related Art
A so-called double voice speaker having two voice coils has been proposed (see U.S. Pat. No. 4,438,297, for example). In such a speaker, a voice coil with a small diameter and a voice coil with a large diameter are coaxially mounted on a diaphragm made of paper or cloth. The diaphragm is integrated with a magnetic circuit comprising a yoke and a magnet fixed to the yoke so that the voice coil with the small diameter and the voice coil with the large diameter are arranged within magnetic gaps in the magnetic circuit. In a speaker with such a structure, the diaphragm is vibrated by both the voice coil with the small diameter and the voice coil with the large diameter to which an audio signal is supplied so as to generate a sound corresponding to the audio signal. With such a speaker, the voice coil with the small diameter has charge of a sound with a comparatively high frequency band while the voice coil with the large diameter has charge of a sound with a comparatively low frequency band, thereby generating a sound with a wide frequency band.
In the speaker with the structure described above, the magnetic gap in the magnetic circuit comprising the yoke and the magnet fixed to the yoke is generally narrow, and the diaphragm must be integrated with the yoke so that the voice coil with the small diameter and the voice coil with the large diameter are appropriately arranged in their respective magnetic gaps. However, the diaphragm made of paper or cloth is liable to deform in the radial direction and in an axial (front-back) direction of the speaker perpendicular to the radial direction. Hence, it is difficult to integrate the diaphragm with the yoke so that the voice coil with the small diameter and the voice coil with the large diameter are appropriately arranged in their respective magnetic gaps.
Also, in the speaker with the structure described above, since the vibration by the voice coil with the small diameter and the vibration by the voice coil with the large diameter are applied to a common diaphragm, the respective vibrations interfere with each other so that a sound with a specific frequency band may be distorted, or a phenomenon may occur in that a resonance is generated so that an acoustic pressure level protrudes at a specific frequency band, as shown within the broken line in FIG. 9.